1-Benzyl-4-[(5,6-dimethoxy-1-indanon)-2-yl]methylpiperidine of formula (I) (hereafter referred to as the compound of formula (I) or Donepezil)
is a well-known compound, and is known to be producible by catalytic hydrogenation of 1-benzyl-4-[(5,6-dimethoxy-1-indanon)-2-ylidene]methylpiperidine of formula (II) (hereinafter referred to as the compound of formula (II)) (see, for example, JP-A-1-79151, Japanese Patent No. 2578475 and Japanese Patent No. 2965675).

JP-A-1-79151 discloses, at page 15, the right lower column and page 17, the left upper column, a reaction scheme represented by the following formulas:

JP-A-1-79151 discloses, at page 16, the left upper, lines 11 to 13, a process of producing a compound of general formula (XXII) by catalytic hydrogenation of a compound of general formula (XXI) by the use of palladium-carbon, Raney nickel or rhodium-carbon.
Furthermore, in Example 4 of JP-A-1-79151, hydrogenation in tetrahydrofuran as solvent for reaction in the presence of 10% palladium-carbon at ordinary temperature and atmospheric pressure is described as an example of reaction.
Japanese Patent No. 2578475 discloses, at pages 3 to 4, a reaction scheme represented by the following formulas:

Japanese Patent No. 2578475 discloses, at page 3, the right column, line 3 from the bottom to the last line, a process of producing the compound of formula (I) by reduction of the compound of formula (II) and describes the fact that when catalytic reduction is carried out, employment of, for example, palladium-carbon, Raney nickel or rhodium-carbon brings about a desirable result. In Example 1 of Japanese Patent No. 2578475, hydrogenation in tetrahydrofuran as solvent for reaction in the presence of 10% palladium-carbon at ordinary temperature and atmospheric pressure is described as an example of reaction.
Japanese Patent No. 2965675 discloses, at page 4, a reaction scheme represented by the following formulas:

Japanese Patent No. 2965675 describes, at page 6, the left column, lines 5 to 19, as follows: a mixture of a compound of formula (II′) and a ruthenium-phosphine complex is dissolved in a solvent such as methylene chloride and the reaction is preferably carried out in an autoclave under the following conditions: hydrogen pressure 4 to 100 kg/cm2, reaction temperature 25 to 75° C., and reaction time 24 to 168 hours. In Example 1 of Japanese Patent No. 2965675, the compound of formula (II′) is reacted in methylene chloride in the presence of a complex [Ru2Cl2((S)-(−)-BINAP)2N(C2H5)3] at a hydrogen pressure of 77 kg/cm2 at 50° C. for 30 minutes and then at room temperature for 140 hours.